1. Field of the Invention
The present invention relates generally to electrical devices for medical application, and more specifically relates to chronically implantable electrodes.
2. Description of the Prior Art
The use of a solid inner stylet to guide the insertion and proper placement of a more flexible coaxial outer catheter is known. Examples of this art may be seen in U.S. Pat. No. 4,020,829, issued to Willson et al. and U.S. Pat. No. 4,215,703, also issued to Wilson. These references teach the guiding of the flexible catheter using such a solid inner stylet wire. To accomplish the desired result, a solid inner stylet wire is given a permanent bend or other shape which causes the catheter to be so bent upon insertion of the inner stylet.
Also common is the use of a stylet for straightening a catheter-like device, which itself has been permanently impressed with a bend or other desired shape. This is particularly prominent in pacing leads or other electrode applications. Examples of electrodes having a predetermined shape may be found in U.S. Pat. No. 3,890,977, issued to Wilson and U.S. Pat. No. 4,136,703, issued to Wittkampf. These references teach the building of an implantable lead with particular reference to a "J" shape for insertion within the atrium of a human heart. The electrodes taught by Wilson and Wittkampf apparently use only the resiliency of the lead itself to maintain contact between the electrode and the stimulatable tissue. Insertion of these "J"-shaped leads is greatly facilitated through the use of a solid inner stylet which, in this case, tends to have a straightened shape for straightening the bend normally fixed within the distal end of the lead.
Leads having positive fixation means are also known in the art. Endocardial leads having a helical fixation means are taught in U.S. Pat. Nos. 4,146,036 4,209,019 and 4,217,913 all issued to Dutcher, et al. Each of these references teaches a body implantable lead for transvenous implantation primarily oriented to implantation within the right ventricle. A stiffening stylet is used to aid in implantation. Torque applied to the stylet is used for fixedly attaching the helical fixation means. In each case, however, the lead must be disposed in a nearly straight fashion and the stiffening stylet used must be nearly straight to enable proper imposition of torque by twisting motion of the stylet.
It has been shown to be desirable to implant a lead such as taught in the Dutcher references discussed above within the right atrium. The most desirable implantation attitude would involve fixation of the helical fixation means in the right atrial appendage. A "J" shape could be imparted to the implantable lead by using a shaped stylet as taught by Willson, et al. in the references discussed above, or by shaping the lead itself as taught in the references by Wilson and Wittkampf. Shaping the stylet would tend to cause difficulty in transmitting torque from the proximal and distal ends of the stylet to enable attachment of the fixation means. In fact, if one would take the stylet taught in the Dutcher references above and impart a "J" shape to it, it would become no longer useful for attaching the helical fixation means.
Shaping the body implantable lead using memory coils or other techniques as taught by Wilson and Wittkampf would allow the stylet to impart the necessary torque. However, this technique is disadvantageous because it makes fabrication of the lead relatively more expensive and increases the handling difficulty because the stylet is normally much stiffer than the body of the implantable lead. This means that the stylet, after being inserted for the purposes of attaching the helical fixation means, would tend to remove the "J" shape from the body of the implantable lead.